The present invention relates generally to teletext communication systems and, more particularly, to improvements in such systems enabling the storage of multiple pages of received teletext formated data by a central station and the automatic rebroadcasting of the stored data by the central station to a plurality of remote receivers at selected times during the day.
Teletext is a generic term for a new and rapidly growing television based communication technique which uses the vertical blanking interval of a broadcast television signal for transmission of text and graphics information. Systems implementing this technique are presently operational in various countries, such as in England where teletext formated text and graphics material is transmitted in digitally encoded form on lines 17, 18, 330 and 331 of their 625 line system. Television receivers equipped with teletext decoders process the transmitted data for forming a display of the text and graphics information. In the British system, which is exemplary of most teletext systems presently in use, the broadcast teletext information is formated in a plurality of cyclically transmitted rows of data, twenty four rows comprising a page of information with each group of one hundred pages representing a magazine. Each row of information, which is digitally encoded for representing up to 40 characters of text or graphics, is serially transmitted on one horizontal line of the vertical blanking interval so that the data transmission rate is two rows per field or four pages per second. In order to provide reasonable access time to the transmitted teletext information, it is customary to limit the cyclically transmitted data to 800 pages. Each teletext decoder conventionally includes a data acquisition circuit responsive to user commands for acquiring a selected page of the recycling teletext information and for storing the twenty four rows of digitally encoded data comprising the page. The stored encoded data is then applied to a display generator which decodes the stored information for synthesizing R, G, B video characters signals which are coupled to and drive the cathode ray tube of the television receiver for producing a display reflecting the stored data, the display comprising a page of twenty four rows of text or graphics with each row consisting of up to 40 characters. The first row, i.e. row 0, of each page is referred to as a page-header and includes an encoded magazine and page number as well as a row address. The remaining rows include magazine and row address but not page number codes. Acquisition of a selected page of data is therefore achieved in the teletext decoder by initially matching the magazine and page number codes of a transmitted page-header row with user selected magazine and page number codes and then stored the matching page-header row together with the next twenty three rows containing the corresponding magazine number.
In the United States, teletext communication techniques are often used for providing selected information to the subscribers of a cable television system. For example, a cable head end operator may receive a number of standard NTSC television signals transmitted by an orbiting satellite for rebroadcasting to the cable system subscribers with teletext encoded data being transmitted during the vertical interval of any one of these signals. The teletext encoded data transmitted by the satellite may represent any desired information including, for example, multiple pages of a programming guide or the like identifying the content of the television programming signals being transmitted by the satellite. The cable head end facility typically includes a conventional teletext decoder which is slightly modified for acquiring and storing the teletext encoded data transmitted by the satellite in a local multiple page memory. Each page of the stored programming guide may then be rebroadcast by the head end facility at selected times of the day as a standard NTSC television signal for display by the television receivers of the cable system subscribers.
It is an object of the present invention to provide a teletext communications system enabling a cable head end operator to continuously acquire and store multiple pages of information transmitted from a remote source, which information may be updated or changed from time to time.
It is another object of the invention to provide a teletext communications system of the foregoing type wherein the multiple pages of information stored at the cable head end facility are automatically rebroadcast thereby at selected predetermined times during the day without any action or intervention being required on the part of the head end operator.
It is yet a further object of the invention to provide a teletext communications system of the foregoing type wherein the cable head end operator is not responsible for maintaining an accurate time base for keying rebroadcasts of the stored information to the cable system subscribers.
It is still another object of the invention to provide a teletext communications system of the foregoing type wherein the multiple pages of information stored at the cable head end facility are automatically rebroadcast to the subscribers in a cyclical manner.